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Informations about the Ordnance Tidal Observatory |
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Thursday, 17 August 2006 |
Somebody told me that it is from Newlyn in Cornwal that sea level and all heights (above sea level) are recorded. Is this true ? And if so, is it valid for world wide measurement (like GMT, for example)?
The following information was obtained by the Newlyn Harbour office:
Based at the South Pier near the lighthouse is the Ordnance Tidal Observatory from which the datum for all the latest ordnance survey maps is defined. Ordnance datum is actually the mean tide level and it’s situated 14.25 feet below the carriageway of the North Pier near the end.
Tidal recordings are taken at the Ordnance Observatory by means of a brass strip with a float arrangement attached. This sets the benchmark for the whole of the United Kingdom and all heights are referenced to this point. The brass strip has holes in it which fits to a sprocket wheel, whilst on a cylinder rotated by a clock is the recording sheet. As on a barograph a pencil arrangement automatically records the inflow and ebb of the tides. As the float rises and falls with the tide the brass strip rotates the sprocket wheel.
The main bench mark is near Tolcarne School and frequent checks are made to determine if there is the slightest shrinkage between the observatory and the main bench mark.
By deducting 9.9 feet from the Ordnance Datum the recordings on the admiralty chart for Mount’s Bay can be arrived by.
Newlyn was chosen as the base for the tidal observatory because, after many years of constantly monitoring the tides, it was found to be the most constant.
The first O.S. selected datum point in 1840 was 100 feet below a bench mark on St. John’s church, Liverpool, and a few years later at the Victoria Docks where tidal observations were taken every five minutes. From 1844 the approximate sea level was adopted from the Liverpool datum and was the accepted reading form 80 years.
Tidal observations were taken at Dunbar, Felixstowe and Newlyn where the first reading was taken in April 1915 when the Pier was extended.
It was decided to base the O.S. datum on the figures from just one tidal observatory and Newlyn, where since 1915 hourly readings had been taken, was chosen as the most suitable.
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1. There is a land tide that effects surveying in britain everytime the water comes flooding back across the planet cornwall goes up and down 25mm. This is averaged out if readings are taken at high and low tides. Land levels can and do change due to earth quakes but not a significant amount for most purposes. Areas with mines can see substantial drops in levels locally. Benchmarks in these areas are ureliable however the OS has not validated most bench marks across england since 1970 and are no longer supported. All levels claiming to be AOD should be GPS derived.
A Datum is a starting point of a co-ordinate system which is in this case a tide gauge, the mean sea level has changed since the original values were observed. Bench marks etc are realisations of this datum.
OSGB36 is now related directly to the european co-ordinate system ETRS89. Because tectonic plate movement is constant and predictable In 1989 Europe was permanently fixed. The Eurasion plate is moving approx 20mm a year but this movemenmt is removed.
2. ITRS - International Terrestial Reference System.
The accademics in the ITRS monitor the locations of the satelites using a range of techniques. This keeps the GPS & GLONASS constellations usable. They also broadcast predicted orbits and very accurate corrected ones 14 days later, others are available in between. Interference from the atmosphere is one of the most significant error sources (Ionosphere). This is removed using mathematical models
3. There is a conversion its called OSGM02 (ordinance survey Ground Model 2002)
GPS derived values can be converted to ODN values using the OSGM02 transformation. OSGM02 is like a rubber sheet it fits the surface of britain.
4. Most countries derive their level datum from a local mean sea level. Sea levels vary around britain and the rest of the world. Britain is in a ocean topography valley approx -80cm below mean ocean levels. Nowadays position & elevation is related to WGS84 GPS readings (actually ITRS & ITRFnot WGS84). Because surveyors are prone to visit pubs for lunch there is a fudge factor every 1km to make the last triagulation fit the more accurate modern GPS methods.....the old triangulation was incredibly good all thing considered.